Raditz cannot suppress his ki. He can power up, like Vegeta does on Earth, but if he could suppress his ki to e.g. 50% then it would not be significant to him and Nappa/Vegeta that the Earthlings can vary their powers wildly.
No one is forced to fight at 100% of their ability from the get go. What impresses them is someone's ability to fool a scouter. The Z-fighters would have been better off with some scouters against the Saiyans compared to their ki sensing abiities.
They got fooled several times. Nappa at full power is a match for 8,000 Goku, yet 1,500 Krillen is able to evade him and land some shots on him. Ergo, Nappa was in no way fighting at full power the entire time.
Raditz when he isn't trying might be showing like 1000 (probably more tbh, going with Vegeta as a benchmark) and then he can power up to his 1200+ level.
He has to be far less than 1,000. Remember, Raditz initial attack against Goku and Piccolo is far superior to what he had shown previously.
Raditz (First Attack) > Raditz (Initial)
You gotta reread the saga. There's quite a few power quotes.
You have Piccolo's Blast vs Initial Raditz that is worth looking into.